1. Field of the Invention
The present invention relates to radio frequency ("RF") power sensors, and in particular, to RF diode power detectors.
2. Description of the Related Art
Radio frequency power sensors, or detectors, are widely used for the sensing and measurement of RF and microwave signal power, both absolute and relative. Traditionally, accurate absolute power measurement has been done by using power sensors having thermal sensing elements in which the input RF signal power is used to heat a resistor, the increase in temperature thereof then being related to the incident RF signal power level. Such power sensors can be very accurate at moderate and high power levels, such as 0.01 milliwatt (-20 dBm) and upwards. However, such power sensors are severely limited in their low end sensitivity, such as below 0.01 milliwatt. Further, such sensors have very slow measurement times, typically on the order of 100 milliseconds.
To overcome these problems, power sensors using semiconductor diodes have become widely used, particularly for relative power measurements. They have the advantages of simple construction, high sensitivity (i.e. capable of measuring RF signal power levels well below 0.01 milliwatt) and very fast response times. However, power sensors using semiconductor diodes have a significant problem in that the range of power levels over which they can be used for precise absolute power measurements is significantly limited due to inherent nonlinear characteristics of semiconductor diodes.
Referring to FIG. 1, a schematic circuit diagram of a first-order model of an equivalent circuit of a semiconductor diode 10 used for RF power detection is illustrated. The diode junction can be modeled by an impedance represented by an equivalent junction capacitance C.sub.J (typically on the order of 0.05 picofarads) and an equivalent junction resistance R.sub.J (typically on the order of 1000 ohms) in parallel therewith. In series with this impedance, is an equivalent epitaxial resistance R.sub.S (typically on the order of 20 ohms). In parallel with all of the foregoing is an equivalent parasitic capacitance C.sub.P (typically on the order of 0.02 picofarads) which represents the effective capacitance due to the physical construction and packaging of the diode 10.
The inherent nonlinear characteristic of the diode 10 of primary concern is due to the junction capacitance C.sub.J. The parasitic capacitance C.sub.P is typically much smaller than the junction capacitance C.sub.J, and therefore has little effect. Significant fractional changes in the junction capacitance C.sub.J cause similar significant changes in the overall impedance of the diode 10. Depending upon other circuit impedances within the circuit (not shown) with which the diode 10 is interacting, the results of this impedance change can introduce significant nonlinear characteristics into the operation of the overall circuit.
Referring to FIG. 2, a schematic circuit diagram of a typical power sensor is illustrated. Shunting the input node to circuit reference, or ground, is a load resistor 30 for absorbing the input RF signal power. The load resistor 30 is typically matched to the line impedance of the circuit or network supplying the input RF signal (e.g. 50 or 75 ohms). Coupling the input and output nodes is the detector diode 20. The diode 20 is conductive on forward RF signal cycles and highly resistive on reverse RF signal cycles. This causes the bypass capacitor 40, shunting the output node to ground, to acquire a positive charge with respect to ground. Ideally, the voltage across the bypass capacitor 40 reaches a steady state value which is approximately equal to the peak value of the RF signal voltage developed across the load resistor 30. Thus, in this steady state condition, the diode 20 is no longer forward biased by the input RF signal, so no further current flows to the output node.
In reality, the characteristics of the diode 20 are not ideal. For small levels of RF input signal voltage, the diode 20 is always conductive, albeit more strongly on forward cycles and less strongly on reverse cycles of the RF input signal. This conductivity of the diode 20 can be approximated by the following formula: EQU I=I.sub.o (e.sup.V/(KT) +1)
where:
I=diode current PA1 I.sub.o =reverse saturation current PA1 e=base of natural logarithms .apprxeq. 2.7182818 PA1 V=diode junction voltage PA1 K=Boltzmann's constant .apprxeq. 8.61.times.10.sup.-5 eV/.degree. K PA1 T=temperature (.degree. K)
Given a few calibration points to determine the constant I.sub.o for the above equation, this equation could be used to predict the output voltage versus input power characteristics of virtually any detector diode. However, the relationship defined by the above formula is accurate for only relatively low RF input signal power levels (e.g. up to 0.01 milliwatt) within what is referred to as the "square law" operating region.
Referring to FIG. 3, a schematic circuit diagram of an improved power sensor circuit is illustrated. This improved power sensor is a "balanced" detector which has a differential output. Greater input signal sensitivity is achieved since the voltages developed across the bypass capacitors 41, 42, via the two diodes 21, 22, are combined to produce a differential output signal. This allows differential amplification of the output signal, which is inherently less noisy, and reduces sensitivity to even harmonics at higher input RF signal power levels.
Referring to FIG. 4, a schematic circuit diagram of an improved balanced detector is illustrated. At RF input signal frequencies in the gigahertz ("GHz") region, the capacitances of the detector diodes 21, 22 and their associated packages tend to shunt the RF load resistance 30. This causes the input voltage standing wave ratio ("VSWR") of the sensor circuit to deteriorate, thereby reducing input signal sensitivity. To reduce the effects of the aforementioned capacitances, "damping resistors" 51, 52 are used in series with the diodes 21, 22 to shield the diodes' capacitances from the circuit or network providing the input RF signal, with little change in input signal sensitivity. Further compensation, e.g. to "tune out" these capacitances, is provided by a tuning inductor 31 connected in series with the RF load resistance 30.
Even with the balanced detector circuit of FIG. 3 or the improved balance detector circuit of FIG. 4, power detection, as represented by the output signal voltage, is still nonlinear with respect to frequency and input RF signal power. Due to these frequency and input RF signal power dependencies, the power detection afforded by these circuits is unpredictable. Correction, or calibration, factors must be introduced which correspond to both numerous input frequencies and numerous input RF signal power levels to compensate for these nonlinearities.
Advances in digital electronic devices and programmable memories has resulted in correction circuits for diode detectors. A typical "corrected" detector circuit includes a resident memory chip having a look-up table for converting the diode's output voltages into a desired linear, or other idealized, response. In principal, the memory needs only to be programmed once by the manufacturer who has access to a suitable means of calibration.
However, due to the aforementioned nonlinear diode characteristics, numerous calibration points are required, particularly if the power sensor is to accurately detect power over a wide range of frequencies and input RF signal power levels. In order to accomplish such correction, or calibration, two "dimensions" of calibration are required: (1) calibration of frequency response since the detected output voltage varies with frequency due to the inherent reactive components of a diode 10, as discussed above; and (2) calibration of voltage output versus input signal power level. True correction would require that calibration factors be stored for all expected input RF signal power levels for all expected frequencies. This would require that a vast amount of data be stored in the resident memory, which in turn, would result in a very slow detector response time due to the need to access the appropriate two-dimensional correction factor each time a measurement is made. This would seriously detract from the most desirable property of a diode detector, namely its fast response time.